1. Field of the Invention
The invention relates generally to the field of oil and gas exploration. More specifically, the present invention relates to an apparatus single point seismic source useful for producing seismic signals.
2. Description of Related Art
Marine seismic exploration generally involves activating a seismic source within a body of water to produce a seismic signal. The signal, which is a sonic pulse or shock wave, propagates through the water and into the geologic formations beneath the body of water. The signal reflects back from the geologic formation as an acoustic wave, which can be recorded by a variety of recording devices, such as geophones, hydrophones, or other like equipment. The recording device or devices can be positioned within the body of water or within a specific wellbore. Positioning the recording devices within a wellbore is known as vertical seismic profiling (VSP). The recorded acoustic wave signal can then be processed and converted into an electronic form for subsequent analysis and interpretation. The result of the analysis provides the structure of the geologic formations under the body of water that can reveal possible hydrocarbon bearing formations. The body of water generally considered here is an ocean, sea, or gulf, but could also include marshes, swamps, lakes, or rivers, or any body having sufficient water depth enabling use of the seismic source.
Many seismic sources are available to produce seismic signals within a body of water. Explosives, such as dynamite, bubble producing capacitors, water guns, and explosive gases have all been used in the past. The current trend however has focused use of devices that discharge compressed gases under the surface of the water. One popular such device is the air gun that is connected to a source of high pressure air having a pressure in the range of from 1000 lbs/in2 to over 6,000 lbs/in2. When fired, the gun releases the high pressure air through a valve in its body; the escaping air expands and forms a large bubble within the body of water. A seismic signal is produced by the action of the air being discharged from the air gun and subsequently expanding.
Since water is a better medium than air for transmitting a seismic signal, the air guns are positioned below the water's surface, generally in the range of 3 feet to 25 feet below the surface of the water. It is preferred to place the air guns from 10 feet to 20 feet below the surface of the body of water. Positioning the air gun at this depth ensures that the bubble produced by the air escaping the air gun fully forms and coalesces before it contacts the water's surface. If the air gun were fired at or just below the water's surface and the resulting bubble did not fully form but instead just blew past the surface of the water, the resulting seismic signal would be less than the seismic signal produced when the bubble does fully form. When the air gun is positioned at a proper depth and the bubble is permitted to form, the size of the bubble will oscillate as it travels towards the water's surface. The bubble oscillations also produce an acoustic signal, which while recordable, is generally much less than the initial acoustic signal produced by the air gun.
The performance, or signature, of an air gun can be recorded by far field measurements during actual use, or can be modeled using computers and specially crafted software. The performance is typically measured by analyzing more than one signature. Some of the possible signatures are an output signature (FIG. 1) and an amplitude spectrum (FIG. 2). Both FIGS. 1 and 2 represent synthetic test data regarding the subject invention. The output signature reflects the seismic magnitude of the original acoustic signal as well as the acoustic signal produced by the bubble oscillation. FIG. 1 is illustrative of the pattern of a typical air gun output signature where the acoustic signals are plotted in terms of a pressure wave (bars @ 1 m or bar m) in relation to time (msec). The peak value of the primary acoustic signal is represented by reference numbers 10 and 12 and the magnitude of the bubble oscillations is represented by reference numbers 14 and 16. The corresponding pressure values from this graph can be easily referenced to determine the output, the peak to peak value, and the peak to bubble ratio. The reference number 12 represents the inverted mirror image of the primary pulse and is created by an almost perfect reflection at the water surface. This reflection is also known as the trough or ghost. The zero to peak value is determined by the value at reference number 10. The peak to peak value is determined by the range from reference number 10 to reference number 12. The reference numbers 14 and 16 refer to the next greatest peak and trough emanating from the oscillating bubble and is well suppressed in this example. The peak to bubble ratio is determined by the ratio of the range 10 to 12 to the range 14 to 16. In an ideal case, the bubble signature would be zero. However this is not practical with an air gun system, so the system is designed to maximize the peak to peak values and minimize the bubble oscillation.
Increasing the output of the signal source in turn increases the depth that the seismic signal penetrates the geophysical formation, thereby increasing the amount of geological information that is subsequently reflected back to the recording devices. As might be expected, it is desired then to maximize the output of the seismic signal in order to obtain as much geological information as possible. However, increasing the output of the primary seismic signal in turn produces larger bubbles that cause a larger bubble oscillation. Thereby reducing the peak to bubble ratio. Because the bubble has its own associated seismic signal, increasing the bubble oscillation increases the bubble seismic signal. The increased bubble seismic signal produces interference with the primary seismic source and can negate the advantage of increasing the output magnitude.
One method of overcoming the interference of the bubble oscillation is to assemble an array of air guns whose combined seismic signature has an increased output without a correspondingly increased bubble seismic signal. Thus by combining two or more air guns in an array an output signal can be increased without decreasing the peak to bubble ratio. Taken one step further, the chamber volumes of the air guns can be varied in size and the physical arrangement and spacing of the air guns can be altered in order to obtain a specific output signature. This process is better known as “tuning.” A discussion and a list of references that discuss tuning air gun arrays can be found in Barber et al., U.S. Pat. No. 4,956,822, which is incorporated by reference herein in its entirety.
Spacing adjacent air guns a certain distance (d) apart can modify the peak output and frequency content of the seismic signal produced by each air gun. This will occur when the value of d is less 12×V1/3, where V is volume of the air gun chamber in cubic inches. When adjacent air guns are spaced at some distance less than d, the seismic signal of the two closely spaced air guns will closely replicate that of a single air gun having a chamber size approximately equal to the sum of the chambers of the individual air guns. When two or more adjacent air guns are located at or closer than this “d” value such that the combined seismic signal mimics a single air gun having the size of the aggregate guns, that particular group of air guns is referred to as a “cluster” of air guns.
One reason to vary the chamber sizes of the air guns is so that the bubbles produced by the air guns of different chamber sizes will have a correspondingly different volume. Because bubbles of different volume oscillate at a different frequency, these multiple sized bubbles oscillating at different frequencies tend to attenuate the bubble oscillation effect through destructive interference. Varying the air gun chamber size also broadens the frequency spectrum over which the seismic signal produces an output. Seismic signals within a higher frequency provide results having higher resolution than lower frequency signals. Whereas lower frequency seismic signals can penetrate deeper within a geological formation than higher frequency seismic signals are able to Since both ends of the seismic signal spectral range produce useful results, it is desired to produce a seismic signal that has output signals across a wide range of low and high frequencies.
Many publications exist regarding the tuning of air gun arrays, however to date many arrays developed are large, cumbersome, and heavy and also require numerous air guns. Not only are these large arrays expensive and difficult to maintain, their large size requires special handling equipment. Further, since most air gun arrays are used in open seas, even normal weather days can provide for a dangerous handling situation. Therefore a need exists for an air gun array that is not only manageable and compact; but also produces a seismic signal having an acceptable peak to bubble ratio with a wide range of output frequencies.
Further, the geological formations of many regions have been analyzed and the underlying formations are currently known, but due to technological constraints that analysis has been performed only to a limited depth. Thus in order to map deeper formations and learn of other potential hydrocarbon producing zones, a need exists for air gun arrays that can produce seismic signals that penetrate deeper into geologic formations than is currently performed.